The aim of this type of mounting is to isolate the engine as fully as possible from the vehicle body in terms of vibration, in order to improve the vibration comfort of the motor vehicle. This involves a conflict in goals between good vibration comfort on the one hand and good noise insulation on the other. Specifically, when the mounting of the engine has a high natural damping, so that the engine does not continue to oscillate after an excursion, but returns without oscillating, the engine noise (i.e., the natural operating noise of the engine between 40-200 Hz) is easily transmitted into the body as a result of which noise comfort is poor. If, on the other hand, the mounting of the engine only has a slight degree of natural damping, so that the engine oscillates following an excursion, this necessarily means poor riding comfort in the motor vehicle since the engine vibrations are subjectively perceived as disturbing. Such engine excursions are usually caused by passage of the vehicle over irregularities in the road which initially force an excursion of the wheel which is spring-mounted with respect to the body, which is then transmitted to the body itself, upon which, finally, the engine is mounted. An excursion of the body with respect to the engine therefore implies an excursion of the engine with respect to the body from which the engine then returns. In this connection, it is important to note that natural engine mount vibrations particularly occur between 8-10 Hz,, which is particularly disturbing to passengers whereas higher-frequency vibrations are perceived only as noise. On the other hand, excursions of the wheel with respect to the body have lower frequencies of about 2-3 Hz, although these are perceived by people simply as inevitable road irregularities and are not perceived as having a significant influence on the vehicle's vibration comfort.
This conflict in goals cannot be resolved in normal course with purely passive hydraulically damped spring elements.
U.S. Pat. No. 3,807,678 has already proposed that the excursion, particularly the acceleration, of a vibration-mounted mass be monitored, and that the damping intensity of the hydraulic spring element be adjusted on that basis. This technique, however, requires that the mass to be vibration-damped be already vibrating, i.e., that it already be in the state of excursion which is to be eliminated in the first place.
Hydraulically damped rubber metal mounts are known, for example, from German patent Nos. 2,905,091 and 3,010,723, in which two separate chambers are connected to one another by means of a throttle valve. When the rubber metal mount is loaded, one of the chambers is compressed so that the hydraulic fluid within it must pass into the other chamber, this flow being restricted by the valve.